Resonator frequency correction by modifying support structures

ABSTRACT

A method including to a resonator coupled to at least one support structure on a substrate, the resonator having a resonating frequency in response to a frequency stimulus, modifying the resonating frequency by modifying the at least one support structure. A method including forming a resonator coupled to at least one support structure on a chip-level substrate, the resonator having a resonating frequency; and modifying the resonating frequency of the resonator by modifying the at least one support structure. A method including applying a frequency stimulus to a resonator coupled to at least one support structure on a chip-level substrate determining a resonating frequency; and modifying the resonating frequency of the resonator by modifying the at least one support structure. An apparatus including a resonator coupled to at least one support structure on a chip-level substrate, the resonator having a resonating frequency tuned by the modification of the at least one support structure to a selected frequency stimulus.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The invention relates to microelectromechanical structures(MEMS).

[0003] 2. Background

[0004] Communication systems generally require partitioning of theelectromagnetic frequency spectrum. Communication transceiver devicestherefore must be capable of high frequency selectivity, i.e., capableof selecting a given frequency band while rejecting all others.Frequency-selective devices, such as filters, oscillators and mixers aretherefore some of the most important components within a transceiver andthe quality of the devices generally dictates the overall architectureof a given transceiver.

[0005] In wireless radio frequency (RF) devices, resonators aregenerally used for signal filtering and generation purposes. The currentstate of the art typically is the use of discrete crystals to make theresonators (off-chip resonators). To miniaturize devices, MEMSresonators have been contemplated.

[0006] In a typical resonator, the resonance frequency after processingis usually different from the targeted value due to processingvariation. For discrete crystals as mentioned above, such resonancefrequency error is usually corrected using laser trimming technology.However, because MEMS resonators (particularly high frequency MEMSresonators) are generally much smaller in size than their crystalcounterparts, traditional laser trimming technology is not a viablealternative. One alternative is to remove or add mass to the resonatorbeam to increase or decrease frequency. However, as beam structures aretargeted to micron or submicron sizes as required for ultra-highfrequency, it is generally impractical to directly modify the beam. Suchmodification to the beam thickness tends to be inaccurate. Theinaccuracy is believed to be principally due to the sensitivity of thespring constant (k) dependency of the beam thickness. Accordingly, whatis needed are techniques to modify the resonance frequency of aresonator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The features, aspects, and advantages of the invention willbecome more thoroughly apparent from the following detailed description,appended claims, and accompanying drawings in which:

[0008]FIG. 1 shows a top perspective schematic view of one example of an“on chip” resonator.

[0009]FIG. 2 shows a schematic, cross-sectional side view of oneembodiment of a MEMS-fabricated resonator.

[0010]FIG. 3 shows a top view of the resonator of FIG. 1.

[0011]FIG. 4 shows the resonator of FIG. 2 following modification of thesupport structure.

[0012]FIG. 5 shows a top view of the structure of FIG. 4.

[0013]FIG. 6 graphically represents the effect of support pad size andthe resonance frequency.

[0014]FIG. 7 shows a schematic, top view of a second embodiment ofMEMS-fabricated resonator.

[0015]FIG. 8 shows a schematic, cross-sectional view of a thirdembodiment of a modified MEMS-fabricated resonator.

[0016]FIG. 9 shows a schematic, cross-sectional side view of a fourthembodiment of a modified MEMS-fabricated resonator.

DETAILED DESCRIPTION

[0017] In one embodiment, a method of modifying the frequency of aresonator is described. Such modification may be suitable, in oneexample, in MEMS-fabricated resonators wherein a MEMS-fabricatedresonator has a frequency that may be a frequency other than thetargeted value due to processing variations in the fabrication of theresonator. To achieve the targeted frequency value, a method ofmodifying the resonator by modifying the support structure is described.An apparatus, such as a resonator, having a frequency determined by themodification of the support structure is also described.

[0018]FIG. 1 schematically illustrates a chip-based resonator structurethat may be used, for example, in a bandpass micromechanical filter.Referring to FIG. 1, structure 100 includes bridge micromechanicalresonator 110. Resonator 110 is coupled at anchors 125A and 125B but isotherwise free-standing. The vibration of the resonator is induced by alateral gap capacitive transducer (a frequency stimulus). The capacitivetransducer is formed by disposing electrode 140 adjacent resonator 110with, in this case, gap 145 between electrode 140 and resonator 110.

[0019] Structure 100 is essentially a two-terminal device having atypical equivalent electrical circuit as shown operates in the followingrepresentative manner. The circuit has two terminals corresponding toelectrode 140 and beam 110, respectively, between which the impedance isvery high at all frequencies other than the resonance frequency. At theresonance frequency, the resistance between the terminals becomes verylow. In the example of use in a filter, an input signal may be passed atthe resonance frequency.

[0020] It is appreciated that one desired performance of a resonatorstructure (such as resonator structure 100) is the ability to limit theresonating frequencies for which the resonator will vibrate and producea mechanical signal. In the case of an oscillator serving, for example,as part of a clock circuit, it is important that the resonator-basedoscillator vibrate at a target clock frequency. In the case of a filter,it may be desirable for a resonator to resonate at target frequenciesand pass an input signal, while not vibrating at other frequencies andthus rejecting other input signal.

[0021] At higher frequencies (e.g., ultra-high frequencies) targetedvibrating frequencies become harder to achieve during fabrication.Continued miniaturization and limits on fabrication technology (e.g.,photolithography) performance contribute to the increased error betweenthe targeted frequency and the actual frequency following fabrication.

[0022]FIG. 2 and FIG. 3 show a schematic, cross-sectional side view of amicro-bridge resonator, such as a resonator used in the assembly shownin FIG. 1. FIG. 2 shows micro-bridge resonator structure 200.Micro-bridge resonator structure 200 is formed on a portion of substrate210. Substrate 210 is, for example, a semiconductor (e.g., silicon)substrate suitable as a base structure for MEMS applications. It isappreciated that other substrates, such as glass (including silicon oninsulator) and ceramic substrates may be suitable. Substrate 210 mayhave contact points (pads, terminals) disposed on its surface to whichdevice structures (e.g., electrodes) may be formed. Conductive tracesmay also be disposed throughout the body of substrate 210 to connectcontact points on the substrate to one another or to another substrate.Substrate 210 may further have one or more device levels, includinginterconnect levels, formed thereon.

[0023] In one embodiment, micro-bridge resonator structure 200 is formedby a series of deposition and etch patterning. Micro-bridge resonatorstructure 200 includes, for example, a polycrystalline silicon bridgeshown, in this view, to comprise support structures 235 overlying anchorportions 220 and beam 240 disposed above electrode 250. Beam 240 is, inone sense, supported between support structure 235 but is otherwisefree-standing. In this embodiment, z-direction thickness of anchorportions 220 of, for example, silicon dioxide (SiO₂), separate beam 240from substrate 210. Electrode 250 is illustrated adjacent beam 240.

[0024] It is appreciated that, once formed, micro-bridge resonatorstructure 200 will have a certain resonating frequency that primarilyowing to the limitations of the processing environment, may or may notbe the targeted frequency for the particular application (e.g., filter,oscillator, etc.). Thus, in certain instances, it is desirable to modifythe resonating frequency of micro-bridge resonator structure 200. Ingeneral, the resonance frequency depends on the length and thickness ofbeam 240. It has also been determined that the resonance frequency maybe influenced by the compliance of the supporting structure.

[0025]FIG. 4 and FIG. 5 show a cross-sectional side view and a top view,respectively, of micro-bridge resonator structure 200 followingmodification of support structures 235 to influence (modify) theresonance frequency of the structure. In this example, notches 260 areintroduced in support structures 235. In one example, notches 260 areformed by lithographic techniques, e.g., patterning a masking materialover micro-bridge resonator structure 200 and etching notches 260 intosupport structures 235. In terms of etching, any suitable etchant toetch, in this case, polycrystalline silicon and the material for anchorportion 220 is suitable. Alternatively, and particularly for reducedfeature size device structures where point control is desired, notches260 may be introduced using a focused ion beam (FIB) or laser etchingprocess. In either case, control of the beam center position can be asgood a few nanometers (nm). Referring to FIG. 4, in this example,notches 260 extend completely through support structures 235 and anchorportion 220.

[0026] It has been found that etching a notch in the support structuresof a micro-bridge resonator generally increases the compliance of thesupport structure. An increase in compliance (flexibility) generallydecreases the resonance frequency of a resonator structure. Complianceis influenced by the effective pad size of the support structures. InFIG. 4, support structures 235 have an effective pad size defined by theparameter D (e.g., y-direction length). Prior to introducing notches260, the pad size is defined by the parameter D₁. After introducingnotches 260, the pad size is defined by the smaller parameter D₂(D₂<D₁). The determination of pad size is defined by parameter D₂ andsupport structure portion 235A while, in this example, support structureportion 235B is mechanically irrelevant.

[0027]FIG. 6 shows the effect of support pad size on the resonancefrequency. The calculation is performed for a resonator beam of 0.25microns (μm) thickness and one micron in length by 1 micron in width. Asillustrated in FIG. 6, as support pad size decreases, for example, fromD₁, to D₂, the resonance frequency decreases.

[0028] As illustrated in the above embodiment, a modification to thesupport structure of a micro-bridge resonator structure can influencethe resonance frequency of the resonator. Thus, in the situation where aMEMS-fabricated resonator does not meet the targeted resonance frequencyfollowing processing (e.g., deposition, patterning), the resonancefrequency may be modified by modifying the support structures. Thus, atargeted value of resonance frequency is attainable by evaluating theresonance frequency of a resonator and modifying the support structures,where necessary, to achieve a desired resonance frequency.

[0029]FIG. 7 shows a second embodiment of a micro-bridge resonatorstructure modified to meet a targeted resonance frequency. Micro-bridgeresonator structure 300 includes support structures 335 supportingresonator beam 340 over a portion of substrate 310. In this view,notches are introduced in support structures 335 about two differentaxes. It has been found that introducing (forming) a notch in thez-direction (e.g., orthogonal to the resonator beam length) is generallymore sensitive to resonance frequency modification than a notch in they-direction. Thus, notches 360 may be introduced (formed) in supportstructures 335 in a z-direction as a “coarse” correction. Such notches360 may be formed as described above with respect to FIGS. 4 and 5 andthe accompanying text. Notches 360 may be formed through a portion,including the entire portion of support structures 335 with thecompliance of beam 340 generally increasing with the depth of the notch.Where notches 360 extend through the entire portion of supportstructures 335, notches 360 and support structures 335 define supportstructure portions 335A and 335B with the pad size defined by parameterD₂ similar to that described above with respect to FIGS. 4 and 5 and theaccompanying text. The coarse modification (correction) may be used, forexample, to bring the resonance frequency within a few one-hundredths ofthe targeted frequency. Thus, having formed notches 360, the resonancefrequency may be evaluated for accuracy with the targeted resonancefrequency.

[0030] To further modify the resonance frequency in a generally minorfashion (e.g., “fine” tuning) additional notches may be introduced insupport structures 335 in a different direction, such as in this case,the y-direction. Referring to FIG. 7, notches 370A and 370B are formedin a y-direction in support structures 335. In this illustration, twonotches 370A and 370B are formed in each support structures. It isappreciated that the number and location of notches will vary dependingon the level of tuning. For example, a single y-direction (or otherdirection) notch may be introduced in each support structures 335 andthis can be followed by an evaluation of the frequency of resonatorstructure 300. If further tuning is necessary, additional (one or more)y-direction (or other direction) notches may be introduced as necessary.

[0031]FIG. 8 shows a third embodiment of a micro-bridge resonatorstructure modified to meet a targeted resonance frequency. Micro-bridgeresonator structure 400 includes support structure 435 supportingresonator beam 440 over a portion of substrate 410. In this view,notches 460 are introduced in support structures 435 in a z-directionthrough less than the entire portion of the support structure. As notedabove, it is appreciated that notches may be formed (for example, by ionbeam or laser cutting technique) through a portion, including the entireportion, of support structures 435 with the compliance of beam 440generally increasing with the depth of the notch. In FIG. 8, notches 460are introduced into support structures 435 to a depth, H. The notches460 modify the pad size of support structures 435 by forming portions435A and 435B. It is appreciated that, where the notches do not proceedthrough the entire portion of support structures 435, support structureportion 435B remains mechanically relevant.

[0032]FIG. 9 shows a fourth embodiment of a micro-bridge resonatorstructure modified to meet a targeted resonance frequency. In thisexample, structural material is added to the support structures to, forexample, increase the target resonance frequency. Micro-bridge resonatorstructure 500 includes support structures 535 supporting resonator beam540 over a portion of substrate 510. It has been found that it ispossible to modify the frequency of vibration by adding material tosupport structures 535. For example, where it is found that theresonance frequency of an on-chip resonator is too low, the resonancefrequency may be increased by adding material 555 to support structures535. It is believed the addition of material 555 to support structures535 stiffens micro-bridge resonator structure 500 thereby increasing theresonance frequency.

[0033] In the embodiment shown in FIG. 9, material 555 is introducedover support structures 535. Suitable material includes but is notlimited to materials having a generally high modulus of elasticity.Materials having a modulus of elasticity of 100 giga-Pascals (gPa) andpreferably greater than 100 gPa are particularly suitable. Suchmaterials include, but are not limited to, silicon nitride (SiN) ortungsten (W).

[0034] In the illustration shown in FIG. 9, a free-standing structure isshown for over substrate 510. In general, a conventional deposition suchas a chemical vapor deposition cannot be used to introduce material 555over support structure 535 due to the possibility that the depositionwill hinder or modify the free-standing structure to a non-free-standingform. Thus, in one embodiment, a point deposition technique is used tointroduce material 555. Such point deposition techniques include laseror focused ion beam deposition techniques.

[0035] In the preceding detailed description, a technique to modify theresonance frequency of a structure (such as a resonator) is described asis structures (resonators) having modified or tuned resonanceproperties. Specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the claims. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. A method comprising: modifying a resonatingfrequency of a resonator coupled to at least one support structure on achip-level substrate by modifying at least one support structure.
 2. Themethod of claim 1, wherein modifying the at least one support structurecomprises removing a portion of the at least one support structure. 3.The method of claim 1, wherein the at least one support structurecomprises a body including a base coupled to the substrate and a topsurface opposite the base, and modifying the at least one supportstructure comprises forming a notch in the top surface.
 4. The method ofclaim 3, wherein forming a notch comprises forming a first notch in afirst direction and a second notch in a second direction.
 5. The methodof claim 1, wherein modifying the at least one support structurecomprises adding material to the at least one support structure.
 6. Amethod comprising: forming a resonator coupled to at least one supportstructure on a chip-level substrate, the resonator having a resonatingfrequency; and modifying the resonating frequency of the resonator bymodifying the at least one support structure.
 7. The method of claim 5,wherein modifying the at least one support structure comprises removinga portion of the at least one support structure.
 8. The method of claim5, wherein the at least one support structure comprises a body includinga base coupled to the substrate and a top surface opposite the base, andmodifying the at least one support structure comprises forming a notchin the top surface.
 9. The method of claim 8, wherein forming a notchcomprises forming a first notch in a first direction and a second notchin a second direction.
 10. The method of claim 8, wherein forming thenotch comprises forming the notch through less than the entire body ofthe at least one support structure.
 11. The method of claim 5, whereinmodifying the at least one support structure comprises adding materialto the at least one support structure.
 12. A method comprising: applyinga frequency stimulus to a resonator coupled to at least one supportstructure on a chip-level substrate; determining a resonating frequency;and modifying the resonating frequency of the resonator by modifying theat least one support structure.
 13. The method of claim 12, whereinmodifying the at least one support structure comprises removing aportion of the at least one support structure.
 14. The method of claim12, wherein the at least one support structure comprises a bodyincluding a base coupled to the substrate and a top surface opposite thebase, and modifying the at least one support structure comprises forminga notch in the top surface.
 15. The method of claim 14, wherein forminga notch comprises forming a first notch in a first direction and asecond notch in a second direction.
 16. The method of claim 14, whereinforming the notch comprises forming the notch through less than theentire body of the at least one support structure.
 17. The method ofclaim 13, wherein modifying the at least one support structure comprisesadding material to the at least one support structure.
 18. An apparatuscomprising: a resonator coupled to at least one support structure on achip-level substrate, the resonator having a resonating frequency tunedby the modification of the at least one support structure to a selectedfrequency stimulus.
 19. The apparatus of claim 18, wherein the at leastone support structure comprises a body, with a base of the body coupledto the substrate and at least one notch formed in a portion of the body.20. The apparatus of claim 19, wherein the at least one notch extendsthrough less than the entire portion of the body.
 21. The apparatus ofclaim 19, wherein the at least one support structure comprises a firstnotch in a first direction and a second notch in a second direction. 22.The apparatus of claim 18, wherein the resonator is coupled at a firstend to a first support structure and at a second end to a second supportstructure.